Variation of global compound heatwaves and their associations with climate variability

Recent widespread heatwaves have broken local temperature records over the world. Large and intense heatwaves not only change the land surface biophysical environment in terms of temperature rise and water shortage, but also endanger our natural and human ecosystems by increasing health risks. Compared to the independent daytime or nighttime heatwave, the compound heatwave often yield higher hot extremes and even pose greater hazard to human and ecosystem health, as it prevents humans or ecosystems recovering from previous hot temperatures if the extreme hot occur in both day and night. However, factors shaping their spatiotemporal patterns on a global scale remain poorly understood, as do their links with large-scale interannual climatic variability. Here, with the air temperatures from multiple global datasets (e.g., ERA5L, CPC, MERRA2, and JRA55), we quantified the frequency and intensity patterns of compound heatwaves over 1980–2019 and analyzed their associations with modes of climate variability. Our results show a significant increasing trend of compound heatwave occurrences on a global scale, with the global average frequency and intensity increased by 90% and 32%, respectively, in 2010–2019 relative to 1980–1989. Specifically, Arctic and mid-latitudes of the Northern Hemisphere have seen the greatest increases in heatwave frequency and intensity over the last four decades, which may be connected to the amplifying influence of Arctic warming as well as human activity. The interannual variability of tropical compound heatwaves is dominated by ENSO occurrences from the previous year to the present year. And, the PDO and AMO modes dominate the interannual variability of extremely high temperatures in the majority of the mid-latitudes. In contrast, the interannual variability of compound heatwaves in the Arctic is most strongly tied to the AO and NAO in the winter of the present year, but it is most directly associated with the AAO and SAM in the Antarctic, which has an 8–9-month lag impact. This work will increase our knowledge of the global patterns and mechanisms of compound heatwaves in a multi-decadal context, hence enhancing our ability to predict hot extremes.